A lithographic apparatus is a machine constructed to apply a desired pattern onto a substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). A lithographic apparatus may, for example, project a pattern (also often referred to as “design layout” or “design”) at a patterning device (e.g., a mask) onto a layer of radiation-sensitive material (resist) provided on a substrate (e.g., a wafer).
To project a pattern on a substrate a lithographic apparatus may use electromagnetic radiation. The wavelength of this radiation determines the minimum size of features which can be formed on the substrate. Typical wavelengths currently in use are 365 nm (i-line), 248 nm, 193 nm and 13.5 nm. A lithographic apparatus, which uses extreme ultraviolet (EUV) radiation, having a wavelength within the range 4-20 nm, for example 6.7 nm or 13.5 nm, may be used to form smaller features on a substrate than a lithographic apparatus which uses, for example, radiation with a wavelength of 193 nm.
Low-k1 lithography may be used to process features with dimensions smaller than the classical resolution limit of a lithographic apparatus. In such process, the resolution formula may be expressed as CD=k1 ×λ/NA, where λ is the wavelength of radiation employed, NA is the numerical aperture of the projection optics in the lithographic apparatus, CD is the “critical dimension” (generally the smallest feature size printed, but in this case half-pitch) and k1 is an empirical resolution factor. In general, the smaller k1 the more difficult it becomes to reproduce the pattern on the substrate that resembles the shape and dimensions planned by a circuit designer in order to achieve particular electrical functionality and performance. To overcome these difficulties, sophisticated fine-tuning steps may be applied to the lithographic projection apparatus and/or design layout. These include, for example, but not limited to, optimization of NA, customized illumination schemes, use of phase shifting patterning devices, various optimization of the design layout such as optical proximity correction (OPC, sometimes also referred to as “optical and process correction”) in the design layout, or other methods generally defined as “resolution enhancement techniques” (RET). Alternatively, tight control loops for controlling a stability of the lithographic apparatus may be used to improve reproduction of the pattern at low k1.
A metrology apparatus, or an inspection apparatus, may be used to determine characteristics of the pattern manufactured on the substrate by the lithographic apparatus. Today many forms of optical metrology technologies are known and with shrinking critical dimensions in the manufactured patterns, these optical metrology technologies may lack resolution. An option is to use in such a metrology apparatus a radiation that has relatively low wavelengths, for example, in the soft X-ray or Extreme Ultraviolet (EUV) spectral range. The relatively low wavelengths may be in the range from 0.1 nm to 100 nm, or in the range from 1 nm to 50 nm, or in the range from 10 nm to 20 nm. One may generate radiation in such wavelengths by using the principle of Higher Harmonic Generation (HHG): short pulses of infrared (IR) radiation are focused in a HHG medium (for example a specific gas) and the HHG medium converts a portion of the received IR radiation towards soft X-ray or EUV radiation. The radiation that is generated by HHG may comprise multiple peaks at different wavelengths in a relatively broad spectrum.
In the metrology apparatus a beam of radiation is directed by an illumination sub-system towards a region of interest on the substrate. At the region of interest is provided, for example, a target. Preferably the beam of radiation is focused on the region of interest or the target.
In the field of satellites and synchrotrons, a few optical systems are known that are capable of focusing broadband radiation in the soft X-ray and/or EUV spectral range on a sensor. Often reflectors are used for reflecting radiation in the soft X-ray and EUV spectral ranges.